Defect fix: MAGN-3605 and MAGN-3646

Pre-compilation of CBN code no longer involves round-tripping from code to AST and back. Once the code is parsed into AST, it's AST all the way. Existing CBN test case regressions show no failures. A patch made to resolve a defect due to this round-tripping in AssociativeAST.cs was reverted with no side-effects. 
Our language parser currently still has limitations parsing certain expressions such as "a+b;" and "a*6;". In order to work around this, we are manually parsing such expressions for now. The strategy is that we first send the code to the main parser and if and only if there is a parser failure, we check for non-assignment expressions such as this and try to manually parse them. It therefore is a 2 stage process in these cases. For most other expressions (including other non-assignment expressions), the main parser is able to handle them, in which case this part of the code isn't touched.